Examination of phytochemical literature reveals that a disproportionately large number of host plants for species in the genus Papilio Section II, and in particular the P. machaon complex, contain linear furanocoumarins. Although the linear furanocoumarin xanthotoxin is known to be toxic to generalist lepidopterous larvae, it failed to affect adversely the growth and survivorship of P. polyxenes, a member of the P. machaon complex that feeds primarily on Umbelliferae containing furanocoumarins, when incorporated into an artificial diet. Growth rate and weight gain were significantly improved in the presence of xanthotoxin. It is proposed, based on the experimental evidence and on host plant utilization patterns, that furanocoumarins play a significant role in the behavioral and biochemical adaptation of Papilio spp. to their umbelliferous hosts.